The type of electrostatic printhead described in WO 93/11866 is well known. Electrostatic printheads of this type eject charged solid particles dispersed in a chemically inert, insulating carrier liquid by using an applied electric field to first concentrate and then eject the solid particles. Concentration occurs because the applied electric field causes electrophoresis and the charged particles move in the electric field towards the substrate until they encounter the surface of the ink. Ejection occurs when the applied electric field creates a force on the charged particles that is large enough to overcome the surface tension. The electric field is generated by creating a potential difference between the ejection location and the substrate; this is achieved by applying voltages to electrodes at and/or surrounding the ejection location.
The location from which ejection occurs is determined by the printhead geometry and the location and shape of the electrodes that create the electric field. Typically, a printhead consists of one or more protrusions from the body of the printhead and these protrusions (also known as ejection upstands) have electrodes on their surface. The polarity of the bias applied to the electrodes is the same as the polarity of the charged particles so that the direction of the electrophoretic force is away from the electrodes and towards the substrate. Further, the overall geometry of the printhead structure and the position of the electrodes are designed such that concentration and ejection occur at a highly localised region around the locations of the protrusions.
The ink is arranged to flow past the ejection location continuously in order to replenish the particles that have been ejected. To enable this flow the ink must be of a low viscosity, typically a few centipoises. The material that is ejected is more viscous because of the higher concentration of particles due to selective ejection of the charged particles; as a result, the technology can be used to print onto non-absorbing substrates because the material will spread less upon impact.
Various printhead designs have been described in the prior art, such as those in WO 93/11866, WO 97/27058, WO 97/27056, WO 98/32609, WO 98/42515, WO 01/30576 and WO 03/101741.
A printhead as described above may, through sustained use, eventually build up deposits of unwanted matter which must be removed. Occasionally, ink particles may form solid deposits in the region of the ejection locations of the printhead and airborne dust particles may settle in the ejection region, including the ejection locations and the intermediate electrode.
A previously known method of removing unwanted matter from the printhead is to pass a cleaning (or rinse) liquid through the ejection region of the printhead in order to expel any debris. The cleaning liquid that is used in such methods is primarily composed of the ink carrier liquid, in which the ink particles are necessarily insoluble. To remove deposits of ink particles that have dried onto any surfaces of the printhead, it is preferable to combine such a method with a mechanical “scrubbing” process. Typically, the mechanical “scrubbing” process involves combining the cleaning liquid with air in order to agitate the flow of the liquid and thereby dislodge ink deposits. In some cases, the effectiveness of this “scrubbing” process has been found to be inadequate at completely removing dried on ink deposits.
Attempts have been made to perform the above method using a cleaning liquid capable of dissolving the ink deposits. However, such liquids were incompatible with the inks used in printing. As it is inevitable that small quantities of cleaning liquid remaining in the printheads after cleaning will mix with the printing ink, the liquids must be compatible with each other.
Other previously known methods that can successfully remove all unwanted matter from a printhead have required manual intervention, either to remove the front face of the printhead to access the ejection region of the printhead, or to remove the printhead from the printing machine such that further cleaning can take place. A wider range of cleaning methods, such as the use of solvent, chemical or ultrasonic baths, may then be applied to the printhead. Any solvent residue can be carefully removed from the printhead before it is reinserted into the printing machine.
The removal, cleaning and subsequent reinsertion of the printhead and/or its front face is a time consuming process that requires significant skill to perform. This necessitates undesirable periods of downtime for the printing machine and increases the risk of damage to elements of the printhead during removal, cleaning and reattachment.
U.S. Pat. No. 6,183,057 B1 teaches an apparatus for cleaning a printer in which a cleaning cap is provided for engagement with the face of a printhead. In use, a continuous flow of a cleaning fluid is passed over the face of a printhead such that viscous forces in the fluid dislodge and remove debris on the printhead. An ultrasonic transducer is provided in order to induce pressure waves having frequencies of approximately 17,000 kHz in the liquid.
Such an apparatus, however, is not suitable for use with printheads comprising ejection regions located behind an intermediate electrode. The constant flow of fluid over the face of the printhead prevents deep penetration of the fluid into a region of the printhead comprising the ejection locations, thus limiting the cleaning action to the exterior of the printhead. The flow of the fluid also results in turbulence which causes an attenuation of the ultrasonic pressure waves when propagating towards the printhead.
Therefore there is a need to provide an improved approach for cleaning a printhead which allows thorough removal of unwanted matter whilst avoiding the problems encountered using known techniques.